This invention relates in general to hermetic sealing of semiconductor chips, and more particularly, to a lid assembly having a selectively deposited integral heat fusible layer for hermetic sealing of a package containing a semiconductor chip.
Integrated circuits in the form of one or more semiconductor chips are assembled for utilization in small packages of various sizes generally known as flat packs. The flat pack encloses the semiconductor chip for protection of the various components therein to prevent damage from handling as well as atmospheric and other destructive conditions. The flat pack is formed of a package having a central opening on one major surface. The semiconductor chip is placed within a cavity of the flat pack defined by the central opening and is connected electrically by leads extending through the walls of the package. It has become conventional to provide a hermetically sealed cover over the central opening to protect the semiconductor chip.
Heretofore, it has been the practice to hermetically seal the semiconductor chip within the cavity of the flat pack by placing a preformed ring of heat fusible material, such as gold-tin eutectic solder, on the sealing area of the package surrounding the cavity, and overlying the central opening with a gold plated Kovar lid. Upon heating the thus formed assembly, the gold-tin eutectic solder forms a hermetic seal between the package and the gold plated lid.
The solder materials of the preforms are generally brittle and their dimensions are so small that the preforms are very fragile and extremely difficult to handle during assembly. Because of the difficulty of handling such preforms, it has also been difficult to insure accurate registration between the preform, the peripheral region of the lid and the sealing area surrounding the cavity of the package. As a consequence, there has been a substantial yield loss in the assembled flat packs due to defects in the hermetic seal. In addition, with the price of gold increasing to unforeseen limits, there is a real economic need to minimize or completely eliminate the use of gold in providing a hermetically sealed flat pack.
One hermetic sealing technique which partially overcomes the above-noted disadvantages is described in U.S. Pat. No. 3,874,549. In general, a gold plated Kovar lid for hermetic sealing of the package is superimposed with a heat fusible preform. The precise registration between the preform and the lid is achieved by spot welding the preform at a plurality of space locations about the peripheral area of the lid.
However, the welding of the preform to the lid greatly increases the risk of trapping dirt and dust therebetween which ultimately causes leaks in the hermetic seal. Visual inspection of the welded preform to the lid will not give any indication of the subsequent quality of the hermetic seal. Accordingly, defects in the hermetic seal are not noticed until the flat pack assembly is fully fabricated, resulting in an additional loss in yield. As these lids with welded preforms are commingled in automatic processing equipment, there is a tendency for the corners of the preforms to lift up and break off, further reducing the overall yield and quality of the hermetic seal.
Thus, there is heretofore an unsolved need to provide a lid for use in hermetically sealing semiconductor devices which overcomes the above-noted disadvantages resulting in the use of known lids and hermetic sealing techniques.